1. Field of the Invention
This invention relates to the recovery of silver from gelatinous photographic and other materials such as silver bearing gelatinous wastes, waste washing water, gelatin emulsion wastes and the like.
2. The Prior Art
In the past gelatin-bound silver has been recovered from waste washing waters from photographic processing by centrifuging. However, a typical centrifuging operation removes only about 77 percent of the silver. "Floccing" has been used in an attempt to recover the remainder, or at least a significant part thereof, but known floccing methods often make it necessary to handle uneconomically large volumes of liquids. Furthermore, the quantity of flocculent must be substantially commensurate with the gelatin content of the wastes for best silver retention. Under these conditions the "floc" will contain a considerable proportion of alumina waste that makes its presence felt during the smelting operation.
In U.S. Pat. No. 1,065,115 there is described a procedure wherein photographic film is treated with enzymes such as pepsin, trypsin or papain to remove the gelatin from the film base which is then recovered. The metallic silver and silver salts are then said to be collected. I have found that this procedure does not accumulate the silver in a sufficiently concentrated form, nor does the silver settle out of the aqueous liquid to an acceptable silver concentration. Also, the decanted supernatant liquid contains more silver compounds in solution than one wants to lose, typically about 100 ppm.